1. Field of the Invention
The invention is related to a transmission system comprising a transmitter for transmitting an input signal to a receiver via a transmission channel, the transmitter comprising an encoder with an excitation signal generator for deriving from a main sequence, a plurality of excitation sequences being parts from the main sequence, said parts being mutually displaced over a plurality of positions, selection means for selecting an excitation sequence resulting in a minimum error between a synthetic signal derived from said excitation sequence, and a target signal derived from the input signal, the transmitter being arranged for transmitting a signal representing an optimal excitation sequence to the receiver, the receiver comprises a decoder with an excitation signal generator for deriving the selected excitation sequence from the signal representing the optimal excitation sequence, and a synthesis filter for deriving a synthetic signal from the optimal sequence of excitation signal samples.
The present invention is also related to a transmitter, an encoder, a transmission method and an encoding method.
2. Description of the Related Art
A transmission system according to the preamble is known from U.S. Pat. No. 5,140,638.
Such transmission systems can be used for transmission of speech signals via a transmission medium such as a radio channel, a coaxial cable or an optical fiber. Such transmission systems can also be used for recording of speech signals on a recording medium such as a magnetic tape or disc. Possible applications are automatic answering machines or dictating machines.
In modern speech transmission systems, the speech signals to be transmitted are often coded using the analysis by synthesis technique. In this technique, a synthetic signal is generated by means of a synthesis filter which is excited by a plurality of excitation sequences. The synthetic speech signal is determined for a plurality of excitation sequences, and an error signal representing the error between the synthetic signal, and a target signal derived from the input signal is determined. The excitation sequence resulting in the smallest error is selected and transmitted in coded form to the receiver.
In the receiver, the excitation sequence is recovered, and a synthetic signal is generated by applying the excitation sequence to a synthesis filter. This synthetic signal is a replica of the input signal of the transmitter.
In order to obtain a good quality of signal transmission a large number (e.g. 1024) of excitation sequences are involved with the selection. This selection involves a large number of filter operations requiring a substantial computing power. In order to reduce the required amount of computing power often a so-called one dimensional codebook is used. This means that the codebook comprises a main sequence of samples from which the excitation sequences are selected. Because adjacent sequences have a large number of samples in common, the filtering can be performed using a recursive method requiring substantially less computational resources. Furthermore, the use of a main sequence from which excitation sequences are selected, results into a reduction of the amount of memory required for storing the excitation sequences. A consequence of the large number of common samples in adjacent sequences is a large correlation value between adjacent sequences. In order to reduce the number of calculations not all possible sequences from the main sequence are used in the coder disclosed in the above mentioned US patent, but only the sequences being mutually displaced over a distance of p sample positions. By doing so some quality loss will inevitably occur.